AD7984BCPZ datasheet(22/24 Pages) AD

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AD7984

Rev. 0 | Page 22 of 24

CHAIN MODE WITH BUSY INDICATOR

This mode can also be used to daisy-chain multiple AD7984s

on a 3-wire serial interface while providing a busy indicator.

This feature is useful for reducing component count and wiring

connections, for example, in isolated multiconverter applications or

for systems with a limited interfacing capacity. Data readback is

analogous to clocking a shift register.

A connection diagram example using three AD7984s is shown

in Figure 38, and the corresponding timing is given in Figure 39.

When SDI and CNV are low, SDO is driven low. With SCK

high, a rising edge on CNV initiates a conversion, selects the

chain mode, and enables the busy indicator feature. In this

mode, CNV is held high during the conversion phase and the

subsequent data readback. When all ADCs in the chain have

completed their conversions, the SDO pin of the ADC closest to

the digital host (see the AD7984 ADC labeled C in Figure 38) is

driven high. This transition on SDO can be used as a busy indicator

to trigger the data readback controlled by the digital host. The

AD7984 then enters the acquisition phase and goes into standby

mode. The data bits stored in the internal shift register are clocked

out, MSB first, by subsequent SCK falling edges. For each ADC,

SDI feeds the input of the internal shift register and is clocked

by the SCK falling edge. Each ADC in the chain outputs its data

MSB first, and 18 × N + 1 clocks are required to read back the N

ADCs. Although the rising edge can be used to capture the data, a

digital host using the SCK falling edge allows a faster reading

rate and, consequently, more AD7984s in the chain, provided

the digital host has an acceptable hold time.

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7984

SDI

SDO

CNV

SCK

AD7984

SDI

SDO

CNV

SCK

IRQ

AD7984

SDI

SDO

CNV

SCK

Figure 38. Chain Mode with Busy Indicator Connection Diagram

SDOA = SDIB

DA17 DA16 DA15

SCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV = SDIA

DA1

417

tSCK

tSCKH

tSCKL

DA0

SDOB = SDIC

DB17 DB16 DB15

DA1

DB1DB0DA17 DA16

tSSDISCK

tHSDISCK

tHSDO

tDSDO

SDOC

DC17 DC16 DC15

DA1DA0

DC1DC0DA16

DB1DB0DA17

DB17 DB16

tDSDOSDI

tSSCKCNV

tHSCKCNV

DA0

tDSDOSDI

Figure 39. Chain Mode with Busy Indicator Serial Interface Timing

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